From a corrosion point of view, the presence of dissolved gases, even in small amounts, is undesirable in water systems which contact metal surfaces. For example, metal surfaces in contact with oxygen-containing industrial water can experience severe pitting. Pitting is highly concentrated corrosion affecting only a small area of the total metal surfaces. This can, however, be a serious problem causing metal failure even though only a small amount of metal is lost and the overall corrosion rate is relatively low.
With respect to oxygen, the severity of attack will depend on the concentration of dissolved oxygen in the water, water pH and temperature. As water temperature increases, as for example in a water heating system, enough driving force is added to the corrosion reaction that small amounts of dissolved oxygen in the water can cause serious problems. Oxygen pitting is considered to be a most serious problem in boiler systems, even where only trace amounts of oxygen are present.
Deaeration is a widely used method for removing oxygen from an oxygen-containing aqueous medium. It is particularly useful for treating boiler feedwater and can be either mechanical or chemical.
While vacuum deaeration has proven to be a useful mechanical deaeration method for treating water distributing systems, boiler feedwater is treated using pressure deaeration with steam as the purge gas. According to the pressure deaeration method for preparing boiler feedwater, the water is sprayed into a steam atmosphere and is heated to a temperature at which the solubility of oxygen in the water is low. About 95 to 99 percent of the oxygen in the feedwater is released to the steam and is purged from the system by venting.
Mechanical deaeration is considered an important first step in removing dissolved oxygen from boiler feedwater. However, as already noted, as water temperature increases, even trace amounts of dissolved oxygen can cause serious problems. Accordingly, supplemental chemical deaeration is often required.
Traditional chemical oxygen scavengers include sodium sulfite and hydrazine. However, sodium sulfite cannot be safely utilized in boiler systems operating at above about 1000-1500 psi as corrosive hydrogen sulfide and sulfur dioxide can be formed at pressures above this range. Also, at these pressures, dissolved solids from the sulfite-oxygen reaction product can become a significant problem.
Hydrazine is a toxic substance and is thought to be carcinogenic. Hence, its use is undesirable.
U.S. Pat. Nos. 4,282,111 (Ciuba) and 4,278,635 (Kerst) both disclose the use of hydroquinone, per se, as an effective boiler water oxygen scavenger. Hydroquinone may be combined with mu-amines, as per U.S. Pat. Nos. 4,289,645 and 4,279,767 (of common assignment herewith) in order to provide a one drum treatment comprising oxygen scavenger and compatible neutralizing amine adapted to inhibit corrosion in the steam condensate system.
The use of hydroquinone, per se, as an oxygen scavenger, although widely accepted, has not totally solved the problem of providing enhanced oxygen corrosion in the steam condensate system. As indicated by the relatively high boiling point of hydroquinone (284.degree. C.), the hydroquinone oxygen scavenger does not readily volatilize to the extent desirable. Accordingly, it mostly remains in the liquid phase and is finally expelled from the system with the blowdown. -n order to provide oxygen based corrosion protection in the steam condensate system, it is desirable to provide an effective chemical oxygen scavenger that readily volatilizes so as to travel with produced steam through the condensate return line to provide oxygen based corrosion protection therein.